The proposed study examines the development of the neural substrate for speech perception between the ages of 4 and 15 years. Although speech perception develops very rapidly during the first year of life, the sensitive period for native-like speech perception closes much later. The first aim, of this proposal is to study second-language speech perception in adults varying in their age of acquisition (AOA) in order to specify the sensitive period for speech perception. The second aim is to understand the changes that result in this decline in plasticity by studying the discrimination of native-language speech sounds, speech in noise, and foreign-language speech sound in children at various points of this sensitive period. The use of fMRI techniques, along with dishabituation paradigms will provide information about the development of the neural substrate for speech perception. Furthermore, these "task-free" paradigms for studying perception also have methodological advantages over other behavioral paradigms used to study speech perception. In particular, the same tasks can be used with child and adult participants, allowing for direct comparison of data from the two specific aims of this proposal.